Wired circuit board

ABSTRACT

A wired circuit board having improved adhesion between the conductive pattern and an insulating layer to prevent a plating solution from remaining between a metal plating layer and the insulating layer. The invention prevents ionic impurities in the plating solution from remaining as residual or ionic contamination, thereby preventing a short circuit from developing when electric current flows through the circuit under a high temperature and high humidity environment. Lower end portions of the terminal portions that are formed on an insulating base layer and lower end portions of side surfaces and metal plating layers that cover the terminal portions are embedded in the insulating base layer in a flexible wired circuit board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application claims of priority from Japanese Patent Appln. No.2004-254852, filed Sep. 1, 2004, the contents of which are hereinincorporated by reference in their entirety.

The present invention relates to a wired circuit board and, moreparticularly, to a wired circuit board used for electronics.

2. Description of the Prior Art

In general, a wired circuit board, such as a flexible wired circuitboard, comprises an insulating base layer, a conductive pattern formedon the insulating base layer, and an insulating cover layer formed onthe insulating base layer to cover the conductive pattern.

This general wired circuit board is required to improve adhesion of theconductive pattern to the insulating base layer to prevent stripping ofthe conductive pattern from the insulating base layer, for improvementin fine pitch of the conductive pattern.

For example, a technique has been proposed to ensure the adhesion of thecircuit to the insulating layer by adhesively bonding a resin filmconsisting primarily of thermosetting resin to a substrate to form aninsulating layer in the B stage state on the substrate, then forming thecircuit on the insulating layer by plating, then embedding the circuitin the insulating layer under pressure, and finally curing theinsulating layer completely to the C stage state (Cf. JP Laid-open(Unexamined) Patent Publication No. 2004-179341, for example).

In this wired circuit board, the conductive pattern is partly exposedfrom the insulating cover layer, and the exposed portion of theconductive pattern is formed as a connecting terminal portion forconnecting to external terminals.

It is known that in order to improve the reliability of the connectingterminal portion connecting to the external terminals or prevent thecorrosion, a metal plating layer, such as a nickel plating layer and agold plating layer, is formed on a surface of the connecting terminalportion (Cf. JP Laid-open (Unexamined) Patent Publication No.2002-185133, for example).

However, in this wired circuit board, there is the possibility when themetal plating layer is formed, a plating solution may infiltrate in aninterface between the metal plating layer and the insulating base layerand remain therein, so that ionic impurities in the plating solution,such as chloride ion, may remain as a residual or ionic contamination.When electric current flows through the circuit under a high temperatureand high humidity environment over a long term in the state of such aresidual remaining, a short circuit may occur from ionic migration, thenleading to insulating failure.

The method disclosed by JP Laid-open (Unexamined) Patent Publication No.2004-179341 as cited above may provide improved adhesion between theinsulating base layer and the conductive pattern, but suffers fromdifficulties in preventing occurrence of the short circuit from theionic migration.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a wired circuit board whicheven when a conductive pattern is formed in the form of fine pitch canprovide improved adhesion between the conductive pattern and aninsulating layer to prevent a plating solution from remaining betweenmetal plating layers and the insulating layer, so as to prevent ionicimpurities in the plating solution from remaining as residual or ioniccontamination, whereby even when electric current flows through thecircuit under a high temperature and high humidity environment over along term, a short circuit from ionic migration can be prevented tosuppress insulating failure.

The present invention provides a wired circuit board comprising aninsulating layer, and a conductive pattern formed on the insulatinglayer, wherein the conductive pattern includes terminal portions, andmetal plating layers are formed on surfaces of the terminal portions,and the terminal portions and the metal plating layers are partlyembedded in the insulating layer.

In the wired circuit board of the present invention, it is preferablethat the metal plating layers are laid over the terminal portions tocover side surfaces and upper surfaces of the terminal portions, and endportions of the terminal portions on the insulating layer side, and endportions of side surfaces of the metal plating layers on the insulatinglayer side are embedded in the insulating layer.

According to the wired circuit board of the present invention, theterminal portions and the metal plating layers are both partly embeddedin the insulating layer. This constituent can provide the result thateven when the conductive pattern is formed in the form of fine pitch,improved adhesion can be provided between the conductive pattern and theinsulating layer to prevent infiltration of a plating solution into aninterface between the metal plating layers and the insulating layer whenthe metal plating layer is formed. This can provide the result ofpreventing the ionic impurities in the plating solution from remainingas residual or ionic contamination in between the metal plating layersand the insulating layer. As a result, even when electric current flowsthrough the circuit under a high temperature and high humidityenvironment over a long term, a short circuit from ionic migration canbe prevented to suppress insulating failure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a flexible wired circuit board presentedas a wired circuit board of the present invention,

-   -   (a) showing a plan view of a principal part of the same,    -   (b) showing a sectional view of the same taken along line A-A′        of (a), and    -   (c) showing a sectional view of the same taken along line B-B′        of (a),

FIG. 2 illustrates a production process of the flexible wired circuitboard shown in FIG. 1,

-   -   (a) illustrating the process of preparing an insulating base        layer,    -   (b) illustrating the process of forming a conductive pattern on        the insulating base layer,    -   (c) illustrating the process of forming an insulating cover        layer,    -   (d) illustrating the process of forming a metal plating layer on        surfaces of respective terminal portions, and    -   (e) illustrating the process of embedding lower end portions of        the respective terminal portions and lower end portions of side        surfaces the respective metal plating layers in the insulating        base layer, and

FIG. 3 illustrates a production process of another embodiment (includingthe step of forming a multilayered insulating base layers) of theproduction process of the flexible wired circuit board shown in FIG. 2,

-   -   (a) illustrating the process of preparing an insulating under        layer,    -   (b) illustrating the process of forming an insulating base layer        by laminating an insulating over layer on the insulating under        layer,    -   (c) illustrating the process of forming a conductive pattern on        the insulating base layer,    -   (d) illustrating the process of forming an insulating cover        layer,    -   (e) illustrating the process of forming a metal plating layer on        surfaces of respective terminal portions, and    -   (f) illustrating the process of embedding lower end portions of        the respective terminal portions and lower end portions of side        surfaces of the respective metal plating layers in the        insulating base layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an embodiment of a flexible wired circuit board presentedas a wired circuit board of the present invention, (a) showing a planview of a principal part of the same, (b) showing a sectional view ofthe same taken along line A-A′ of (a), and (c) showing a sectional viewof the same taken along line B-B′ of (a).

A flexible wired circuit board 1 is formed in a longitudinally extendingband plate form and comprises, as shown in FIG. 1( b), an insulatingbase layer 2 presented as an insulating layer, a conductive pattern 3formed on the insulating base layer 2, and an insulating cover layer 4formed on the insulating base layer 2 to cover the conductive pattern 3.

As shown in FIG. 1( a), one longitudinal end portion of the insulatingbase layer 2 is formed in a generally rectangular form, when viewed fromtop, on which the conductive pattern 3 is formed as a pattern configuredby a plurality of wires 3 a, 3 b, 3 c, and 3 d.

The wires 3 a, 3 b, 3 c, and 3 d are extended to a location near onelongitudinal end of the flexible wired circuit board 1 along alongitudinal direction of the flexible wired circuit board 1. Also,these wires are spaced apart from each other at a predetermined distanceand arranged in parallel along a widthwise direction of the flexiblewired circuit board 1 (a direction orthogonal to the longitudinaldirection thereof).

The insulating cover layer 4 is formed on the insulating base layer 2 tocover over the wires 3 a, 3 b, 3 c, and 3 d extending from a locationspaced apart from the one longitudinal end of the flexible wired circuitboard 1 toward the other longitudinal end thereof.

In other words, the insulating cover layer 4 is not formed in a regionextending between the one longitudinal end of the flexible wired circuitboard 1 and the one longitudinal end of the insulating cover layer 4, sothat the insulating base layer 2 and the wires 3 a, 3 b, 3 c, and 3 d inthat region are exposed from the insulating cover layer 4. The exposedportions of the wires 3 a, 3 b, 3 c, and 3 d are presented in the formof terminal portions 5 of generally rectangular form when viewed fromtop.

Also, metal plating layers 6 are formed on respective surfaces of theterminal portions 5, i.e., an upper surface, both widthwise sidesurfaces, and one longitudinal side surface of each terminal 5, all ofwhich are exposed from the insulating cover layer 4.

Then, in this flexible wired circuit board 1, lower end portions (endportions on the insulating base layer 2 side) of terminal portions 5 andlower end portions (end portions on the insulating base layer 2 side) ofside surfaces (both widthwise side surfaces and one longitudinal sidesurface) of the metal plating layers 6 covering over the terminalportions 5 are embedded in the insulating base layer 2.

Next, a production method of this flexible wired circuit board 1 will bedescribed with reference to FIG. 2. Also, the process of embedding thelower end portions of the terminals 5 and the lower end portions of theside surfaces of the metal plating layers 6 in the insulating base layer2 will be described in detail.

In this method, the insulating base layer 2 is prepared, first, as shownin FIG. 2( a). No particular limitation is imposed on the material usedfor the insulating base layer 2 of the flexible wired circuit board 1,as long as the material may be used for the insulating base layer 2. Forexample, a synthetic resin in film form, such as polyimide resin,polyamideimide resin, acrylic resin, polyether nitrile resin, polyethersulfonic resin, polyethylene terephthalate resin, polyethylenenaphthalate resin, and polyvinyl chloride resin, can be used as theinsulating base layer 2. Preferably, material having a glass-transitiontemperature (Tg) of 150-300° C. is used in terms of workability. Ofthese resin films, a polyimide resin film is preferably used in terms ofheat resistance. The insulating base layer 2 has a thickness of e.g.5-50 μm, or preferably 10-30 μm.

Then, the conductive pattern 3 is formed on the insulating base layer 2,as shown in FIG. 1( b). No particular limitation is imposed on theformation of the conductive pattern 3. A known patterning process suchas an additive process and a subtractive process, may be used forforming the conductive pattern 3.

In the additive process, a thin metal film serving as a seed film isformed on the entire surface of the insulating base layer 2, first.Preferably, the thin metal film can be formed by a thin film formingprocess, such as a sputtering process, using chromium, nickel, copperand alloys thereof. Then, a plating resist of a reverse pattern to theconductive pattern 3 is formed on a surface of the thin metal film. Theplating resist may be formed by a known process using a dry filmphotoresist. Thereafter, the conductive pattern 3 is formed on a surfaceof the insulating base layer 2 exposed from the plating resist. Theconductive pattern 3 can be formed from copper and the like byelectrolytic plating. Thereafter, the plating resist is removed byetching or by stripping and then the thin metal film exposed from theconductive pattern 3 is removed by etching.

In the subtractive process, a metal foil, such as a copper foil, islaminated on the entire surface of the insulating base layer 2, using,if necessary, an adhesive layer, first. A known two-layer base materialproduced previously laminating the insulating base layer 2 on the metalfoil may be used. Then, an etching resist having a corresponding patternto the conductive pattern 3 is formed on the metal foil. The etchingresist can be formed by a known method using a dry film resist. Then,the metal foil exposed from the etching resist is etched. Thereafter,the etching resist is removed by etching or by stripping.

The conductive pattern 3 is formed in the form of a pattern comprisingthe plurality of wires 3 a, 3 b, 3 c, and 3 d by the process mentionedabove. A width of each wire 3 a, 3 b, 3 c, 3 d is in the range of e.g.10-200 μm, or preferably 15-50 μm, and a distance between adconnectingwires is in the range of e.g. 10-200 μm, or preferably 15-50 μm. Also, athickness of the conductive pattern 3 is in the range of e.g. 3-50 μm,or preferably 5-20 μm.

Then, the insulating cover layer 4 is formed, as shown in FIG. 2( c).

The insulating cover layer 4 is formed from the same synthetic resin asthat of the insulating base layer 2. For example, epoxy resins orurethane resins may be used for forming the insulating cover layer 4.Among the synthetic resins cited above, a photosensitive resin, orpreferably a photosensitive polyimide precursor resin (polyamic acidresin), is preferably used for the insulating cover layer 4.

The insulating cover layer 4 can be formed in the following processes.For example, varnish of the photosensitive resin is coated over theentire surface of the insulating base layer 2 including the conductivepattern 3, first. Then, the varnish coated is exposed to light through aphoto mask and then is developed so that it is patterned so that theinsulating base layer 2 and the conductive pattern 3 can be exposed fromthe varnish in the one longitudinal end portion of the flexible wiredcircuit board 1 (in other words, so that the insulating cover layer 4cannot be formed in the one longitudinal end portion of the flexiblewired circuit board 1).

Then, the varnish is dried and then cured by heating. This can producethe result that the insulating cover layer 4 for covering the conductivepattern 3 is laid over the insulating base layer 2 not to cover the onelongitudinal end portion of the flexible wired circuit board 1.

Alternatively, the insulating cover layer 4 may be formed by using, forexample, a previously trimmed synthetic resin film, instead of thephotosensitive synthetic resin. In this alternation, the previouslytrimmed synthetic resin film is adhesively bonded onto the insulatingbase layer 2 including the conductive pattern 3 through an adhesivelayer, if necessary, not to cover the one longitudinal end portion ofthe flexible wired circuit board 1.

The insulating cover layer 4 is formed to have a thickness of e.g. 3-50μm, or preferably 5-30 μm, (including a thickness of the adhesive layer,if any).

As a result of this, exposed portions of the wires 3 a, 3 b, 3 c, 3 d ofthe conductive pattern 3 exposed from the insulating cover layer 4 inthe one longitudinal end portion of the flexible wired circuit board 1are presented in the form of the terminal portions 5 of a generallyrectangular form when viewed from top.

Then, the metal plating layer 6 is formed on surfaces of each of theterminals 5, i.e., a top surface, both widthwise side surfaces, and onelongitudinal side surface of each of the terminals 5 which are allexposed, as shown in FIG. 2( d).

The metal plating layer 6 is formed of e.g. gold or nickel and is formedby plating, such as electrolytic plating or electroless plating.Preferably, the electroless gold plating or the electroless nickelplating is used for forming the metal plating layer 6. When the metalplating layer 6 is the gold plating layer, it has a thickness of e.g.0.1-1 μm, while on the other hand, when the metal plating layer 6 is thenickel plating layer, it has a thickness of e.g. 0.5-5 μm.

Then, in this method, the lower end portions of the each terminalportion 5 and the lower end portions of the side surfaces of the eachmetal plating layer 6 covering the each terminal portion 5 are embeddedin the insulating base layer 2 in a thickness direction thereof, asshown in FIG. 2( e). The flexible wired circuit board 1 is produced inthe method mentioned above.

The embedment of the lower end portions of the each terminal portion 5and the lower end portions of the side surfaces of the each metalplating layer 6 in the insulating base layer 2 is accomplished by hotpressing from the top of the metal plating layer 6 covering the eachterminal portion 5 toward the insulating base layer 2. For example, theconditions for the hot pressing are such that the temperature is a glasstransition temperature or more, or preferably 150-350° C., and thepressure is in the range of 0.2-10 kN/cm², or preferably 0.3-5 kN/cm².

An embedment depth of the lower end portions of the each terminalportion 5 and the lower end portions of the side surfaces of the eachmetal plating layer 6 in the insulating base layer 2 is e.g. 1 μm ormore and the embedment ratio is 50% or less of the thickness of theinsulating base layer 2. When the embedment depth is less (shallower)than that, there is the possibility that a short circuit from ionicmigration may occur. On the other hand, when the embedment depth is more(deeper) than that, there is the possibility that in the embeddingprocess, the terminal portions 5 may penetrate the insulating base layer2 in the thickness direction or the insulating base layer 2 may bedeformed.

In the flexible wired circuit board 1 thus produced, the lower endportions of the each terminal portion 5 are embedded in the insulatingbase layer 2 so that they can be situated on the thicknesswise inside ofthe insulating base layer 2. This constituent can provide the resultthat even when the conductive pattern 3 is formed in the form of finepitch, improved adhesion can be provided between the conductive pattern3 and the insulating base layer 2.

Also, in this flexible wired circuit board 1, not only the lower endportions of the each terminal portion 5 but also the lower end portionsof the side surfaces of the each metal plating layer 6 covering the eachterminal portion 5 are embedded in the insulating base layer 2 so thatthey can be situated on the thicknesswise inside of the insulating baselayer 2. This constituent can provide the result that even when theconductive pattern 3 is formed in the form of fine pitch, a platingsolution can be prevented from infiltrating into between the metalplating layers 6 and the insulating base layer 2 in the process offorming the metal plating layers 6. This can provide the result ofpreventing the ionic impurities in the plating solution from remainingas residual or ionic contamination in between the metal plating layers 6and the insulating base layer 2. As a result, even when electric currentflows through the circuit of this flexible wired circuit board 1 under ahigh temperature and high humidity environment over a long term, a shortcircuit from ionic migration can be prevented to suppress insulatingfailure.

In the processes illustrated in FIG. 2, the insulating base layer 2 maybe formed from two or more layers different in glass transitiontemperature from each other. When the insulating base layer 2 is formedfrom two or more layers different in glass transition temperature fromeach other, the lower end portions of the each terminal portion 5 andthe lower end portions of the side surfaces of the each metal platinglayer 6 can be embedded in the insulating base layer 2 with improvedaccuracy of amount of embedment.

This flexible wired circuit board 1 is formed in the followingprocesses. First, an insulating under layer 7 having a higher glasstransition temperature is prepared as the insulating base layer 2, asshown in FIG. 3( a). Then, an insulating over layer 8 having a lowerglass transition temperature is laminated on the insulating under layer7 to form the insulating base layer 2, as shown in FIG. 3( b).

The insulating under layer 7 is formed of a synthetic resin having aglass transition temperature higher than a glass transition temperatureof a synthetic resin used for forming the insulating over layer 8. To bemore specific, a synthetic resin having a glass transition temperatureof e.g. not less than 250° C., or preferably not less than 280° C., isused for the insulating under layer 7.

The insulating over layer 8 is formed of a synthetic resin having aglass transition temperature lower than a glass transition temperatureof a synthetic resin used for forming the insulating under layer 7. Tobe more specific, a synthetic resin having a glass transitiontemperature of e.g. 150-300° C., or preferably 150-250° C., is used forthe insulating over layer 8.

The insulating over layer 8 is laminated on the insulating under layer 7by using a casting process, for example. The insulating under layer 7and the insulating over layer 8 are formed so that the total thicknessthereof can be substantially equal to the thickness of the insulatingbase layer 2 mentioned above. The insulating over layer 8 is formed tohave a thickness larger than a depth corresponding to an amount ofembedment of the lower end portions of the each terminal portion 5 andthe lower end portions of the side surfaces of the each metal platinglayer 6 in the insulating base layer 2. To be more specific, thethickness of the insulating over layer 8 is preferably set to be 5% to50% of the thickness of the insulating base layer 2 so that physicalproperties of the insulating base layer 2 can be held by the insulatingunder layer 7.

Then, the conductive pattern 3 is formed on the insulating base layer 2in the same manner as in the process shown in FIG. 1 (Cf. FIG. 3( c))and, thereafter, the insulating cover layer 4 is formed (Cf. FIG. 3(d)). Then, the metal plating layers 6 is formed on the respectiveterminal portions 5 (Cf. FIG. 3( e)) and, thereafter, the lower endportions of the each terminal portion 5 and the lower end portions ofthe side surfaces of the respective metal plating layers 6 for coveringthe terminal portions 5 are embedded so that they can be situated on thethicknesswise inside of the insulating over layer 8 of the insulatingbase layer 2, as shown in FIG. 3( f). The flexible wired circuit board 1is produced in the manner mentioned above.

It is preferable in the embedding process that the embedment is carriedout by hot pressing at a temperature in the range of between not lessthan the glass transition temperature of the synthetic resin used forforming the insulating over layer 8 and less than the glass transitiontemperature of the synthetic resin used for forming the insulating underlayer 7. This can produce the result that the lower end portions of theeach terminal portion 5 and the lower end portions of the side surfacesof the each metal plating layer 6 can be embedded in the insulatinglayer extending from a front surface of the insulating over layer 8 to afront surface of the insulating under layer 7 so that they can besituated on the thicknesswise inside of the insulating over layer 8 inthe insulating base layer 2 with improved reliability and improvedaccuracy of amount of embedment.

The flexible wired circuit board 1 thus produced can provide improvedadhesion between the conductive pattern 3 and the insulating base layer2, as is the case with the above. Also, the plating solution can beprevented from infiltrating into between the metal plating layers 6 andthe insulating base layer 2 in the process of forming the metal platinglayers 6. As a result of this, even when electric current flows throughthe circuit of this flexible wired circuit board 1 under a hightemperature and high humidity environment over a long term, a shortcircuit from ionic migration can be prevented to suppress insulatingfailure.

Although the wired circuit board of the present invention created as asingle sided flexible wired circuit board has been illustrated above,the wired circuit board of the present invention is applicable to adouble sided flexible wired circuit board and is also applicable to arigid-flexible wired circuit board and a rigid wired circuit board.

EXAMPLES

While in the following, the present invention will be described infurther detail with reference to Examples and Comparative Examples, thepresent invention is not limited to any of Examples and ComparativeExamples.

Example 1

An insulating base layer formed by a polyimide resin film having athickness of 25 μm and a glass transition temperature of 230° C. wasprepared, first (Cf. FIG. 2( a)).

Then, a thin metal film was formed on the insulating base layer bysputtering a thin chromium film having a thickness of 0.03 μm and a thincopper film having a thickness of 0.10 μm sequentially. Then, a platingresist of a reverse pattern to the conductive pattern was formed on thethin metal film. Thereafter, a conductive pattern formed from copper wasformed by electrolytic copper plating on a surface of the insulatingbase layer exposed from the plating resist. Thereafter, the platingresist was stripped and then the thin metal film exposed from theconductive pattern was removed by wet etching. As a result, theconductive pattern having a thickness of 10 μm was formed in a patterncomprising a plurality of wires which were spaced from each other at adistance of 30 μm (Cf. FIG. 2( b)).

Then, varnish of polyamic acid resin was coated over the entire surfaceof the insulating base layer including the conductive pattern. Then, thevarnish coated was exposed to light through a photo mask and then wasdeveloped so that it was patterned so that the insulating base layer andthe conductive pattern (terminal portions) could be exposed from thevarnish in the one longitudinal end portion of the flexible wiredcircuit board. Thereafter, the varnish was dried and then cured byheating and thereby an insulating cover layer, formed of polyimide resinand having a thickness of 15 μm, for covering the conductive pattern wasformed on the insulating base layer (Cf. FIG. 2( c)).

Then, a gold plating layer having a thickness of 0.5 μm was formed onsurfaces of each terminal portion (top surface, both widthwise sidesurfaces, and one longitudinal side surface) (Cf. FIG. 2( d)).Thereafter, the terminal portions were subjected to hot pressing from atop of the gold plating layer covering the each terminal portion towardthe insulating base layer at the temperature of 250° C. and the pressureof 0.3 kN/cm² for ten seconds. The flexible wired circuit board wasproduced in the manner mentioned above (Cf. FIG. 2( e)).

In the flexible wired circuit board obtained, an amount of embedment oflower end portions of each terminal portion and lower end portions ofside surfaces of each gold plating layer in the insulating base layerwas 2 μm.

Example 2

An insulating base layer formed by laminating an insulating over layerformed by a polyimide resin film having a thickness of 5 μm and a glasstransition temperature of 230° C. on an insulating under layer formed bya polyimide resin film having a thickness of 20 μm and a glasstransition temperature of 350° C. was prepared (Cf. FIG. 3( b)).

Then, a conductive pattern having a thickness of 10 μm was formed in apattern comprising a plurality of wires spaced from each other at adistance of 30 μm in the same manner as in Example 1 (Cf. FIG. 3( c)).

Further, an insulating cover layer, formed by a polyimide resin having athickness of 15 μm, for covering the conductive pattern was formed onthe insulating base layer in the same manner as in Example 1 (Cf. FIG.3( d)).

Then, a gold plating layer having a thickness of 0.5 μm was formed onsurfaces of each terminal portion in the same manner as in Example 1(Cf. FIG. 3( e)). Thereafter, the terminal portions were subjected tohot pressing from a top of the gold plating layer covering the eachterminal portion toward the insulating base layer in the same manner asin Example 1. The flexible wired circuit board was produced in themanner mentioned above (Cf. FIG. 3( f)).

In the flexible wired circuit board obtained, an amount of embedment oflower end portions of each terminal portion and lower end portions ofside surfaces of each gold plating layer in the insulating base layerwas 2 μm.

Comparative Example 1

Except that the process of the hot pressing from a top of the goldplating layer covering the each terminal portion toward the insulatingbase layer was omitted, the same processes as those in Example 1 weretaken to produce a flexible wired circuit board.

Comparative Example 2

Except that the process that after formation of the insulating coverlayer, the terminal portions were subjected to hot pressing from a topof the terminal portions toward the insulating base layer at thetemperature of 250° C. and the pressure of 0.3 kN/cm² for ten secondsand then gold plating layers each having a thickness of 0.5 μm wereformed on surfaces (a top surface, both widthwise side surfaces, and onelongitudinal side surface) of each terminal portion, the same processesas those in Example 1 were taken to produce a flexible wired circuitboard.

In the flexible wired circuit board obtained, the lower end portions ofthe side surfaces of the gold plating layers were not embedded in theinsulating base layer, and only the lower end portions of the respectiveterminal portions were embedded and the amount of embedment was 2 μm.

Evaluation

After the flexible wired circuit boards of Examples and those ofComparative Examples were placed under high temperature and highhumidity environment of 60° C. and 95% RH, electric current flowedthrough the circuit at an applied voltage of 30V. Electricity wasmeasured until an insulation resistance value was reduced to 10⁶ Ω orless (until a short circuit occurred from ionic migration) and electricreliability was evaluated by the elapsed time. The results are shownbelow.

-   -   Example 1 Elapsed time: 800 hours    -   Example 2 Elapsed time: 800 hours    -   Comparative Example 1 Elapsed time: 600 hours    -   Comparative Example 2 Elapsed time: 400 hours

While the illustrative embodiments of the present invention are providedin the above description, such is for illustrative purpose only and itis not to be construed restrictively. Modification and variation of thepresent invention that will be obvious to those skilled in the art is tobe covered by the following claims.

1. A wired circuit board comprising: an insulating layer, a conductive pattern formed on the insulating layer, wherein the conductive pattern includes terminal portions, and metal plating layers that are formed on surfaces of the terminal portions, wherein a lower end portion of side surfaces of the terminal portions and a lower end portion of side surfaces of the metal plating layers are embedded in the insulting layer and wherein an upper end portion of side surfaces of the metal plating layers is not embedded in the insulating layer.
 2. The wired circuit board according to claim 1, wherein the metal plating layers are laid over the terminal portions to cover side surfaces and upper surfaces of the terminal portions.
 3. A wired circuit board, comprising: an insulating base layer; a conductive pattern formed on the insulating base layer; and an insulating cover layer formed on the insulating base layer to cover the conductive pattern, wherein the conductive pattern has a terminal portion that is exposed from the insulating cover layer, wherein a metal plating layer is formed on a surface of the terminal portion; and wherein a lower end portion of the terminal portion and a lower end portion of the side surface of the metal plating layer covering over the terminal portion are embedded in the insulating base layer. 